I agree with Airpuka. I’m not sure if a TESOL Master’s will license you to teach in places where more widely based teaching qualifications are required for registration as a teacher. You’d need to check that. Some education authorities will require a teacher education qualification in addition to the Master’s in TESOL before registering someone as a teacher. However, if you can get registration either before or after completing the TESOL Master’s, the TESOL qualification can be quite useful.
When I was hiring people in Australia as literacy and learning support teachers quite a few candidates had a TESOL qualification and they were often the best candidates. Likewise, people teaching languages other than English and regionally based consultants or advisory teachers for language and literacy education often had a TESOL qualification. It was a useful thing to have even if you were not working in the TESOL field specifically. The understanding of language and language pedagogy gained in a postgraduate TESOL course was valued by employers. I’m thinking of Graduate Diplomas and Master’s degrees, though, rather than Cambridge and Trinity courses, useful as they may be for teaching ESOL. (Also, all the people I'm referring to had general teaching qualifications and experience as well.)
Still, if you don’t have a particular interest in language teaching and learning, TESOL may not be the best way to go as a long term career choice. In fact, if you don’t have a real fondness for teaching, a qualification for something other than teaching is probably best.
Incidentally, all the qualified teachers I know with TESOL qualifications and experience have been able to diversify quite broadly. They've certainly not had to spend their lives in a classroom unless they wanted to. Those who like being permanent classroom teachers have been able to make stable and satisfying school-based careers for themselves. They love teaching.